In vehicles, evaporative fuel which leaks into the ambient air from a fuel tank, etc., is described as one of causes of air pollution because of the large content of hydrocarbons (HC). The evaporating fuel also contributes to fuel loss. Accordingly, various techniques are known as a prevention thereagainst, and there is an evaporative fuel controller representative of one such technique. More specifically, in this controller, evaporated fuel from the fuel tank is absorbed by a canister which contains an absorbent such as activated carbon. When an engine is run, then the absorbed fuel is released from the canister so as to be supplied to the intake system of the engine.
Examples of the above-described controller are disclosed in published Japanese Patent Application Laid-Open Nos. 5-321774 and 6-117336, as detailed below.
One evaporative fuel controller as disclosed in the 5-321774 publication provides a purge control method in which, when evaporative fuel absorbed by the canister is purged into the intake system of an engine on the basis of a purge duty ratio during engine idle operation, then a weight loss correction factor for correcting a quantity of fuel in accordance with a purging amount of evaporated fuel is set to execute addition and subtraction between the correction factor and a predetermined value, thereby controlling an air-fuel ratio so as to purge the evaporated fuel; and, the purge control method is characterized by the steps of: setting two different predetermined values as the predetermined value; determining a concentration of evaporated fuel during purging thereof; and, shifting the aforesaid predetermined values from one to another, depending upon a higher or lower level of the determined concentration, thereby executing addition and subtraction between the correction factor and the predetermined value.
Another evaporative fuel controller as disclosed in the 6-117336 publication includes a control means for inhibiting the supply of evaporated fuel to the intake system of the engine when the control means detects that the engine is in idle operation.
In the prior art evaporative fuel controllers, when the fuel tank is supplied with fuel during operation of the internal combustion engine, a large amount of fuel vapor, which is generated in the fuel tank, is discharged together with air into the ambient air through the canister. At the same time, the evaporative fuel controller is controlling a purge valve because the engine is running. Thus, evaporated fuel is supplied from the canister to the intake system. This means that the controller supplies a high concentration of evaporated fuel to the intake system when the fuel tank is supplied with fuel during operation of the engine. This causes problems in that the engine operates poorly and further undergoes rough idle operation, with a consequential stall.